The present invention relates to compositions which are convertible to components useful in numerous applications. More particularly, the subject invention provides compositions comprising a polymeric material reinforced with electrically conductive composite fibers which are convertible to form conductive components.
Fibers of non-metals and semi-metals, such as carbon, boron, silicon carbide, titanium dioxide and the like, in the form of filaments, whiskers, mats and cloths are known to be useful in reinforcing organic polymeric materials. Articles comprising plastics reinforced with such fibers find wide-spread application in replacing heavier components made of lower strength conventional materials, such as aluminum, steel, titanium, vinyl polymers, nylons, polyesters, etc., in aircraft, automobiles, office equipment, sporting goods, and in many other fields. In many of these applications it has become increasingly desirable to provide electrically conductive, otherwise electrically functional and thermally conductive reinforced polymeric compositions which are convertible to form many useful articles or components for articles.
In the past, attempts at rendering polymeric materials electrically conductive have met with varying degrees of success. Loading polymers with carbon fibers has been tried, but in order to achieve acceptable electrical conductivity, so much carbon fiber must be added that there is a marked decrease in certain other desirable polymeric properties. Silver or other metal flakes and metal coated glass spheres have also been added to polymers yet, again, very high loading levels are needed to achieve electrical conductivity which becomes cost prohibitive for most applications and a decrease in polymer properties has likewise been observed. Another problem with the metal coated spheres is that the metal-sphere bond strength is not high enough to prevent peeling and flaking of the metal from the spheres during normal polymer processing, such as high speed mixing, which results in decreased electrical conductivity in the final product.
Numerous unsuccessful attempts have been reported to provide fibers and filaments, and especially carbon filaments with an electrically conductive metal coating. In the past, thin metallic surface layers were deposited on the filaments by vacuum deposition, e.g. U.S. Pat. No. 4,132,828; electroless deposition; and electrolytic deposition, e.g. U.S. Pat. No. 3,622,283. Each of these methods failed to produce composite fibers having a satisfactory bond strength of metal coating to fiber so that when these fibers were bent and twisted as occurs during the processing of reinforcements and polymer materials, i.e., braiding, chopping, weaving, high shear mixing, paint roll mixing, extrusion, etc., the metal coatings would crimp, buckle, flake, and fall off the fiber core. Thus when these prior art fibers were incorporated into polymer compositions, they failed to provide a desirable level of continuous and uniform electrical conductivity. The prior art composite fibers were not satisfactory primarily because boundary layers present on the carbon filaments prevented satisfactory bonding of the metal to the filament. The poor adhesion of the metal is not only a processing problem, but seriously interferes with the ultimate end use, such as, for example, loss of conductivity of said articles when exposed to vibration and shock.
These boundary layers are formed during the preparation of high strength carbon fibers. The carbon fibers are made by heating polymeric fibers, e.g., acrylonitrile polymers or copolymers, in two stages, one to remove volatiles and carbonize and another to convert amorphous carbon into crystalline carbon. During such procedure, it is known that the carbon changes from amorphous to crystals , then orients into fibrils. If the fibers are stretched during the graphitization, then high fiber strengths are formed. This is implicit in the formation of the boundary layer, because as the crystals grow, there are formed high surface energies, as exemplified by incomplete bonds, edge-to-edge stresses, differences in morphology, and the like. It is also known that the new carbon fibrils in this form can scavenge oxygen from the air, and even organic materials, to produce non-carbon surface layers which are firmly and chemically bonded thereto. Some of the boundary layer impurities can be removed by solvent treating, but not always completely. In any case, the boundary layers generally interfere with the formation of bonds between the metal and the inmost fibril.
It has recently been disclosed in copending application, Ser. No. 358,637 now abandoned, filed concurrently herewith, that new and improved composite fibers comprised of carbon filaments having uniform, continuous adherent thin metal coatings thereon may be prepared by electroplating the fibers, if a very high order of external voltage is applied. The voltage must be high enough to provide energy sufficient to push the metal ions through the boundary layers to provide contact with the fibrils directly. The bond strength of the metal to the core is not substantially less than about ten percent of the tensile strength of the metal, so that these composite fibers may be bent with neither transverse cracking on the compression side of the bend, nor breaking and flaking on the tension side of the bend when the elastic limit of the metal is exceeded. These fibers can be woven into cloths, yarns, mats, and the like, and can be folded and knotted without the metal flaking off. They are described as being compatible reinforcement for plastics and metals.
It has now been discovered that the composite fibers described in said above-mentioned application, are capable of being incorporated into a polymeric material which may then be converted to components exhibiting satisfactory electrical conductivity and wide utility. Electrical and thermal conductivity may be provided by relatively low concentrations of composite fibers and, therefore, interference with other desirable polymer properties is minimal or non-existent.